My dad has a Multimaster and it's truly awesome.
Like any tool, there are jobs it is good for.
There are quite a few tasks I've found where it would have been really useful - mainly in repairs and renovations, where you aren't ripping out whole floor boards or walls or anything. Those are things where a reciprocating saw, jig saw, circular saw or even a dremel tool (a cutting wheel is always the wrong angle.) won't work well.
He used it to cut out windows of my daughter's playhouse after we put up the plywood walls, and it doesn't do a great job on long cuts like that - he was cutting blind from the inside along the studs and the cuts aren't perpendicular to the sheet (but it's covered with trim).
(I actually asked at least partly so I could answer - so here goes):
I bought one of the cheap Harbor Freight variety on sale, and I can see where it would do a great job. Small cuts and getting into corners are some things that it's great at. The example on some of the commercials about trimming some door frame to put in flooring is a perfect example of one thing it's great at. One thing that's especially worth a mention though is that it still has some arc to the end of the cutting path. Not much, but some. The regular blade like this:
is one of the best ones.
I've already used it to put together several projects. It does a fair job at cutting dovetails - though it doesn't tend to cut super straight through 2x4.
The "safety" part is pretty nice - you can touch the tip of the blade without getting your finger ripped off, but you wouldn't want to leave your finger there.
I've also used the triangular sander and that seems to be really nice - I can see where the carbide piece would be great for removing tile cement. If I am ever involved in that project you can bet I'll be using this oscillator.
One thing I've noticed - you really don't often notice how completely ridiculous this tool looks - it feels like holding a really long "normal" can. It doesn't look or feel like any normal tool (drill, saw, etc) that's designed to be held. For the oscillator it's more of an after thought - but honestly probably the best form for the tool.
Another thing I've discovered is that turning the cutting tool seems to be basically useless - my cuts aren't as straight or easy. The only time it's really good is when using one of those moon/rounded blades.
All in all I'd say the $34 Harbor Freight version belongs in everyone's toolbox. But I probably wouldn't get one of the more expensive varieties unless I were on a job where I was constantly using it.
I have the harbor freight version, and it has served me well, but I would definitely recommend the upgrade to one of the more mainstream versions.
I've been through 3 so far, all warranty replacements, for switch failures. Even with the failures, I've been very happy with the job that the tool did and will definitely upgrade to a Fein if and when we purchase the fixer upper house we are looking for.
Once you have one of these, you'll wonder how you did without one. I got one for undercutting door jambs to make room for flooring, but have ended up using it for a lot of little tasks I never even thought about.
The ability to make accurate plunge cuts is invaluable. With a carbide blade, I have used one to cut through plaster wall without the remaining plaster cracking or crumbling. I even used this to cut through a vent pipe in a wall where I no clearance for a reciprocating saw (though, that required 3 carbide blades and a fair amount of time).
Very useful for finishing a circular saw cut where clearance prevents you from use the circular saw for the entire cut. Good sander especially in tight spaces or where control is important. This list goes on and on honestly....
The Fein unit is too expensive IMHO, the Harbor Freight I've never used. I frankly don't trust the cheap harbor freight stuff, but it's true, for the price I guess it doesn't matter if it doesn't last long. I myself got a Ridgid from Home Depot for about $100. Came with blades and a carry bag. Now that I'm hooked on Ryobi's One+ line, my only regret is that the tool isn't cordless. I may pick up one... Overall, one of my favorite tools for its versatility... it just gets used.